I’m standing on a catwalk in a giant cave crammed with industrial equipment, and I’m told that trillions of neutrinos are flying through every inch of my body each second. I reach out my arms as if to heighten the sensation, but of course, I can’t feel a thing. Nearly massless, traveling close to the speed of light, the ghostly particles traverse the empty space between my atoms without a trace. They also move mostly unimpeded through the hulking metal box that dominates the cavern. But a few times a day one will collide with an atom inside the school bus–size contraption, liberating charged particles that leave light trails visible to scientists. And these trails, physicists hope, will lead them into unknown territory.

The apparatus is part of the NuMI Off-Axis Electron Neutrino Appearance experiment, or NOvA, here at Fermi National Accelerator Laboratory (Fermilab) in Batavia, Ill. A similar but larger detector is buried 800 kilometers away in Minnesota, where it catches neutrinos that have passed through this one and all the ground in between. NOvA, which has been operating since 2014, is the world’s longest-distance neutrino experiment, but it is laying the groundwork for something much larger—the Deep Underground Neutrino Experiment (DUNE). DUNE will start at Fermilab, where an accelerator will speed up and smash protons into graphite to create a beam of neutrinos. Those neutrinos will then fly through 1,300 kilometers of earth from Illinois to South Dakota. The additional 500 kilometers of travel should make it more likely that the neutrinos will display some of their trademark odd behavior.

DUNE is the most ambitious particle physics experiment to be attempted on U.S. soil since the failed Superconducting Super Collider (SSC) of the 1990s. The $1.5-billion project is scheduled to start up in the 2020s and should run for at least 20 years. But it is not just Americans who are excited—the project involves more than 1,000 researchers from 31 countries and counting. It will be the biggest neutrino experiment on the planet. It will also mark the first time that Europe’s major particle physics laboratory, CERN, has ever invested in a project outside the continent. Just as the Large Hadron Collider (LHC) discovered the famed Higgs boson in 2012, revealing the presence of a hidden field that fills the cosmos, scientists hope DUNE can use neutrinos to understand the universe on a deeper level. “We want to do for neutrinos what the LHC did for Higgs,” says DUNE’s former co-spokesperson Mark Thomson, an energetic Brit from the University of Cambridge, who helped lead the charge on the experiment. (He is currently executive chair of the U.K.’s Science and Technology Facilities Council.) “We believe we are on the verge of launching the next major revolution in particle physics.”

Neutrinos stoke such extravagant hopes because they are the first particles to break from the so-called Standard Model, physicists’ best description of nature’s fundamental particles and the rules that govern them. The Standard Model, which explains the behavior of every other known particle with extraordinary precision, predicts that neutrinos should be massless. And that’s what scientists thought until around 20 years ago, when experiments in Japan and Canada discovered that neutrinos do have the slightest bit of mass. But neutrinos don’t seem to acquire mass the way other particles do. Instead, it appears, they come by their heft through so-called new physics—some particle, force or phenomenon that scientists have not yet found.

Over the past few years neutrinos have come to look like an ever more promising bridge to the future of physics because other attempts to reach that frontier have come up short. So far the LHC has failed to produce any particles not predicted by the Standard Model. Experiments designed to reveal the particles that make up dark matter, the invisible stuff that dominates the cosmos, have also come up empty. “We know the Standard Model is not complete—there are other things going on, but we don’t know what,” says Fermilab neutrino physicist Stephen Parke. “Some people are betting on the LHC with their careers. Others of us are betting on neutrinos.”

Massive mystery

The day after my visit to the NOvA cave, I find myself sitting in an empty office on the third floor of Robert Rathbun Wilson Hall, Fermilab’s main building. Parke, who is here along with theorist André de Gouvêa of Northwestern University, says he chose this room for our meeting because it was once the office of the late Leon Lederman, the former director of Fermilab, who developed a way to create a beam of neutrinos with a particle accelerator. That work, the bedrock of DUNE, revealed the existence of one of the three known types of neutrinos in 1962 and later won Lederman a Nobel Prize. Parke and de Gouvêa admit that although the field has come a long way since Lederman’s day, scientists are still puzzled. “The thing about neutrinos is, the more you understand, the more questions you have,” Parke says. “They’re very mischievous particles.”

Parke, a native of New Zealand, got hooked on neutrinos shortly after coming to the U.S. for graduate school in the 1970s. In the subsequent decades, neutrinos lost their reputation as massless, boring particles. “There have been these revolutions one after the other,” he says. “The question is, Are there more revolutions out there?” He and de Gouvêa are betting yes. “We’ve only just begun to measure neutrino properties at a level comparable to other particles,” de Gouvêa says. “We don’t know their masses, there could be new [types of neutrinos], the neutrinos could talk to other particles that don’t talk to anybody else.”

DUNE will focus on neutrinos’ bizarre tendency to swap identities, a process called oscillation. The particles come in three varieties, or flavors: electron neutrinos, muon neutrinos and tau neutrinos. Researchers can tell them apart because when they interact with atoms in detectors, they produce different end products—electron neutrinos create electrons, muon neutrinos produce muons and tau neutrinos make tau particles (muons and taus are heavier cousins of electrons). Strangely, these three flavors are mutable. The particles might leave Fermilab as muon neutrinos and arrive in South Dakota as electron neutrinos. Or they might show up as tau neutrinos. As far as physicists know, neutrinos are the only particles that undergo this bizarre act of identity transformation.

Credit: Don Foley (DUNE schematic) and Jen Christiansen (neutrino primer)

When physicists discovered the shape-shifting tendency of neutrinos almost two decades ago, it solved a long-standing mystery. In the 1960s, when scientists began studying neutrinos streaming out of the sun, they measured only about a third of the output predicted by theory. Oscillation explained why: the missing two thirds were morphing from electron neutrinos into muon and tau neutrinos as they traveled to Earth, but the instruments were set up to see only electron neutrinos. Although the discovery put to bed the so-called solar neutrino problem, it exposed another mystery: according to theory, the only way for neutrinos to switch flavors is for them to have mass—and that is something that the Standard Model did not predict.

The reason physicists know neutrinos must have mass is a head-scratcher that comes from quantum theory. For neutrinos to change flavors, each flavor must be made up of different “mass states.” Weirdly, each neutrino flavor does not appear to have a definitive mass; instead the flavors are a mix of three possible masses. (If that sounds strange, blame quantum mechanics, which tells us that particles are not definite entities but uncertain hazes of probability.) As neutrinos fly through space, the parts associated with each mass state travel at slightly different rates, a consequence of Einstein’s special theory of relativity, which established that the velocity of a particle traveling near the speed of light depends on its mass. Over time this difference is thought to cause the mixture of masses in each neutrino to change, so a particle that starts out as, say, a muon neutrino, defined by its precise mass mixture, can turn into an electron or tau neutrino.

Scientists still do not know what the precise neutrino mass states are—only that they are different and nonzero. But by counting how many neutrinos oscillate during the journey from Illinois to South Dakota, DUNE aims to determine how the different neutrino masses compare with one another. Theory suggests that the three possible neutrino masses might be ordered so that two are very lightweight and one is heavy or, alternatively, that two of the masses are heavy and one is smaller. The first of these two options is known as the normal hierarchy, whereas the second arrangement is called the inverted hierarchy. DUNE should be able to distinguish between the two because the matter inside Earth is thought to affect neutrino oscillations; if the normal hierarchy were correct, scientists would expect to see different ratios of the three flavors than if the inverted hierarchy were right. “By firing neutrinos through matter, you can determine that difference very easily, and the farther you fire your neutrinos, the clearer your signal is,” Thomson says. “That’s a bit of physics that DUNE is absolutely guaranteed to nail within a few years.”

Fermilab's main injector, an underground particle accelerator ring, ramps up protons to create beams of neutrinos to be studied by the DUNE experiment. Credit: Courtesy of Department of Energy and Fermilab

The origin of mass

Once they know the ordering of the neutrino masses, researchers can tackle the larger question of how neutrinos get their mass. Most particles, such as the protons and neutrons inside atoms, acquire mass by interacting with the Higgs field; this field, which pervades all of space, is associated with the Higgs boson found at the LHC. But the Higgs mechanism works only on particles that come in both right-handed and left-handed versions, a fundamental difference related to the orientation of their spin relative to their direction of motion. So far neutrinos have been seen only in left-handed form. If they got mass from the Higgs field, then right-handed neutrinos must also exist. But right-handed neutrinos have never been observed, which suggests that if they are real they do not interact at all with any other forces or particles in nature—and that prospect strikes some physicists as far-fetched. Furthermore, if the Higgs field did work on neutrinos, theorists would expect them to have similar masses to the other known particles. Yet neutrinos are inexplicably light. Whatever the mass states are, they are less than one hundred-thousandth of the mass of the already puny electron. “Very few people think it’s the Higgs mechanism that gives mass to the neutrinos,” says Fermilab’s director Nigel Lockyer. “There’s probably a completely different mechanism, and therefore there should be other particles associated with how that happens.”

One possibility that excites physicists is that neutrinos could be Majorana particles—particles that are their own antiparticles. (This is possible because neutrinos have no electric charge, and it is a difference in charge that distinguishes a particle from its antimatter counterpart.) Theorists think Majorana particles have a way of getting mass without involving the Higgs field—perhaps by interacting with a new, undiscovered field. The mathematics behind this scenario also requires the existence of a very heavy set of neutrinos that has yet to be discovered; these particles would have up to a trillion times the mass of some of the heaviest known particles and would, in a sense, counterbalance the light neutrinos. For particle physicists, the prospect of discovering a new mass scale is enticing. “Historically we’ve always made progress by exploring nature at different scales,” de Gouvêa says. And if some new field gives mass to neutrinos, maybe it affects other particles as well. “If nature knows how to do it to neutrinos, where else does it do it?” Lockyer speculates. “Theorists are asking: Could dark matter be a Majorana mass?”

DUNE will not directly test whether neutrinos are Majorana particles, but by measuring the mass hierarchy, it will help scientists interpret the results of experiments that do, which are going on now in Japan, Europe, the U.S. and elsewhere. Plus, DUNE should help elucidate the origin of neutrino mass by providing details about how neutrinos switch between mass combinations during oscillation. “We want to do the best possible neutrino oscillation experiment,” de Gouvêa says, “because that’s the one place where we know we’re going to learn something about neutrino masses.”

Matter vs. antimatter

Probing the oddities of these minuscule particles could also help solve a mystery of cosmic proportions: why the universe is made of matter and not antimatter.

Cosmologists predict the two should have existed in equal amounts after the big bang. Somehow, after most of the matter annihilated with most of the antimatter (as the two do on contact), there was a slight excess of matter left over. That matter makes up the galaxies, stars and planets that we see today.

To account for this asymmetry, scientists are on the lookout for a type of particle that behaves differently from its antimatter counterpart, and various clues, including hints seen at other experiments, point to neutrinos. DUNE will search for signs of so-called CP (charge parity) violation—in other words, evidence that antineutrinos oscillate from flavor to flavor at different rates than neutrinos. For example, theory suggests that DUNE might see antimatter muon neutrinos turning into electron neutrinos at anywhere between half to twice the rate at which matter neutrinos make this transition—a difference that Parke calls “enormous” and that could explain why matter won out in that initial battle. (Bizarrely, neutrinos could still oscillate differently from antineutrinos even if the two turn out to be same thing—in other words, if neutrinos are Majorana particles. In that case, the only thing separating neutrinos from antineutrinos would be their handedness, related to their direction of spin. Matter neutrinos, being left-handed, could act differently from antimatter neutrinos, which would be right-handed.)

DUNE will also be able to determine whether neutrinos come in only three flavors or whether there are more waiting to be discovered, as some theories speculate. The additional neutrino flavors would be so-called sterile neutrinos because they would not interact with normal matter at all. Earlier experiments, including the Liquid Scintillator Neutrino Detector at Los Alamos National Laboratory and the Mini Booster Neutrino Experiment (MiniBooNE) at Fermilab, saw inconclusive signs that an extra type of neutrino was interfering with oscillations, suggesting that sterile neutrinos exist that are heavier than the regular three. Researchers hope DUNE will either confirm or rule out that possibility. “Sterile neutrinos can change the pattern of oscillations we see at DUNE by quite a large amount,” Thomson says.

Betting big

To address all these quandaries, scientists designed DUNE to collect far more data at far greater levels of precision than every previous neutrino experiment. The project will use a beam of neutrinos about twice as powerful as the strongest existing high-energy neutrino stream, and it will blast it at a detector that is more than 100 times larger than the biggest of its kind.

The centerpiece of the experiment will be the far detector to be installed in the Sanford Underground Research Facility in Lead, S.D. That machine will consist of four detector modules, each as long as an Olympic pool but six times as deep, that will be filled with 17,000 metric tons of liquid argon. When a neutrino strikes the nucleus of an argon atom in either the far or near detector, it will become, depending on its flavor, an electron, a muon or a tau particle. Muons will travel through the liquid argon in straight lines, kicking electrons out of argon atoms as they go, leaving a trail of electrons the detector can see. If the neutrino produces an electron, on the other hand, the process will create a photon that will then spawn two electrons, and then more photons, and so on, in a cascade of new particles. Tau neutrinos, likewise, would result in tau particles but only if the initial neutrino was energetic enough; taus, being more massive than electrons or muons, take more energy to create. Scientists at CERN began testing a miniature version of DUNE’s far detector, called ProtoDUNE, in 2018. “These detectors, it’s kind of like a space mission in that once you turn them on you really can’t stop them and take them apart to fix things,” says Joseph Lykken, Fermilab’s deputy director. “Once you put the 17,000 tons of liquid argon in, it’s just too hard to get it out.”

To succeed, DUNE will have to overcome the political and funding hurdles that have killed large physics projects before. In July 2017 scientists and officials held a groundbreaking ceremony at the Sanford facility to mark the start of major excavation, which is still ongoing. Of course, plenty of excavation took place for the SSC, which was planned to be even bigger than the LHC. The SSC probably would have discovered the Higgs boson, but it was canceled in 1993 because of cost overruns and changing political tides. “You can go back in history and look at the Supercollider, and, boy, is that a sad story,” Lockyer says. “The international nature of DUNE is such a step forward.” Having commitments and funding from more than just one country should help DUNE avoid the SSC’s fate. “I’ll say it’s definitely happening,” Lockyer says. And then he catches himself: “But could it not happen? Yes.”